Flexible board and electronic device

ABSTRACT

A flexible board includes a first sheet section including a first principal surface, a second sheet section including a second principal surface and provided in a different position from the first principal surface in a normal direction to the first principal surface, at least one first bent sheet section configured to connect ends of the first and second sheet sections, the first bent sheet section including a third principal surface not parallel to the first and second principal surfaces, and at least two second bent sheet sections each including a fourth principal surface and provided in different positions from the third principal surface in a normal direction to the third principal surface. The second bent sheet sections are positioned so as to sandwich the first bent sheet section therebetween when viewed in a plan view in the normal direction to the third principal surface.

This application is based on Japanese Patent Application No. 2013-001098filed on Jan. 8, 2013 and International Application No.PCT/JP2013/081418 filed on Nov. 21, 2013, the entire contents of each ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flexible board bent at predeterminedpositions and an electronic device including the same.

2. Description of the Related Art

A conventional flexible board of this type is described in, for example,International Publication No. WO2012/073591. As shown in FIG. 11, theflexible board 50 described in International Publication No.WO2012/073591 includes a flexible base 501 and a plurality of externalterminals 503.

The base 501 includes a plurality of dielectric sheets. Each dielectricsheet is made of flexible, thermoplastic resin such as polyimide orliquid crystal polymer. Moreover, the dielectric sheet, when viewed in aplan view in a direction z (referred to below as a normal direction z)parallel to a line normal to its principal surface, is in the shape of arectangle extending in a direction x perpendicular to the normaldirection z. The base 501 is formed by laminating the dielectric sheetsin the normal direction z. In addition, signal lines and groundconductors are provided in or on the base 501 during the course oflamination.

The external terminals 503 are rectangular conductors positioned atopposite ends of the base 501 in the perpendicular direction x, and usedas, for example, signal terminals or ground terminals. These conductorsare made of a metal material, preferably, a metal foil, mainly composedof silver or copper and including a low specific resistance.

The flexible board 50 is used for electrically connecting twohigh-frequency circuits within the housing of an electronic device bythe external terminals 503. In the housings of electronic devices ofrecent years, various components and modules are integrated with highdensity. To place the flexible board 50 in such a housing, the flexibleboard 50 is bent in accordance with the shape of the space in which itis to be placed. In the example of FIG. 11, the flexible board 50 isbent at four portions A to D, resulting in a first sheet section 505 a,second sheet sections 507 a and 507 b, and bent sheet sections 509 a and509 b.

The first sheet section 505 a is a center portion of the flexible board50, and is positioned at a different level from the second sheetsections 507 a and 507 b in the normal direction z. A first end of thefirst sheet section 505 a in the perpendicular direction x (namely, bendB) is connected to a first end of the bent sheet section 509 a. Inaddition, a second end of the first sheet section 505 a (namely, bend C)is connected to a first end of the bent sheet section 509 b.

Furthermore, the second sheet section 507 a is positioned on thenegative side of the flexible board 50 in the perpendicular direction x,and is typically fixed within the housing. The second sheet section 507a has at least one of the external terminals 503 provided at its firstend in the perpendicular direction x, and is connected at a second end(namely, bend A) to a second end of the bent sheet section 509 a.

Furthermore, the second sheet section 507 b is positioned on thepositive side of the flexible board 50 in the perpendicular direction x,and is typically fixed within the housing. The second sheet section 507b has at least one of the external terminals 503 provided at its firstend in the perpendicular direction x, and is connected at a second end(namely, bend D) to a second end of the bent sheet section 509 b.

The bent sheet section 509 a is connected between the first sheetsection 505 a and the second sheet section 507 a so as to beperpendicular thereto. The bent sheet section 509 b is connected betweenthe first sheet section 505 a and the second sheet section 507 b so asto be perpendicular thereto.

However, depending on the bending manner, high-frequency characteristics(in particular, isolation) might vary between individual flexible boards50 housed in a plurality of electronic devices. For example, the bentsheet sections 509 a and 509 b are flexible and extend in the normaldirection z, and further, the first sheet section 505 a is connected ontop of both of them. Accordingly, as the size of the first sheet section505 a increases, the shape of the bent sheet sections 509 a and 509 bbecomes unstable. As a result, for example, the signal lines in the bentsheet section 509 a might be positioned closer to the signal lines inanother sheet section or an external high-frequency circuit, causingcrosstalk between them. In addition, the first sheet section 505 a mightbe deflected, for example, under its own weight, causing crosstalk.

SUMMARY OF THE INVENTION

A flexible board according to a preferred embodiment of the presentinvention includes a first sheet section including a first principalsurface, a second sheet section including a second principal surface andprovided in a different position from the first principal surface in anormal direction to the first principal surface, at least one first bentsheet section configured to connect ends of the first and second sheetsections, the first bent sheet section including a third principalsurface not parallel to the first and second principal surfaces, and atleast two second bent sheet sections each including a fourth principalsurface and provided in different positions from the third principalsurface in a normal direction to the third principal surface. The secondbent sheet sections are positioned so as to sandwich the first bentsheet section therebetween when viewed in a plan view in the normaldirection to the third principal surface.

An electronic device according to another preferred embodiment of thepresent invention includes the flexible board according to the abovepreferred embodiment and at least two high-frequency circuits connectedby the flexible board.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view illustrating a flexible board according to apreferred embodiment of the present invention.

FIG. 2 provides top views of the flexible board shown in FIG. 1 (beforeand after bending).

FIG. 3 is an exploded view illustrating in enlargement a portion shownin a dotted circle of FIG. 2.

FIG. 4A is a side view illustrating the internal configuration of anelectronic device to which the flexible board shown in FIG. 1 isapplied.

FIG. 4B is a top view illustrating the internal configuration of theelectronic device shown in FIG. 4A.

FIG. 5 is an oblique view illustrating a flexible board according to afirst modification of a preferred embodiment of the present invention.

FIG. 6 provides top views of the flexible board in FIG. 5 (before andafter bending),

FIG. 7 is an oblique view illustrating a flexible board according to asecond modification of a preferred embodiment of the present invention.

FIG. 8 provides top views of the flexible board in FIG. 7 (before andafter bending).

FIG. 9 is an oblique view illustrating a flexible board according to athird modification of a preferred embodiment of the present invention.

FIG. 10 provides top views of the flexible board in FIG. 9 (before andafter bending).

FIG. 11 is an oblique view illustrating a conventional flexible board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an oblique view illustrating the configuration of a flexibleboard 1 according to a preferred embodiment of the present invention.FIG. 2 provides top views of the flexible board 1 shown in FIG. 1, wherethe upper portion shows the flexible board 1 before bending, and thelower portion shows the flexible board 1 after bending. FIG. 3 is anexploded view illustrating in enlargement the structure of the flexibleboard 1 in circle α of FIG. 2.

In FIGS. 1 to 3, the x-axis represents the right-left direction(longitudinal direction) of the flexible board 1. The y-axis representsthe front-rear direction of the flexible board 1. The z-axis representsthe top-bottom direction of the flexible board 1. Further, the z-axisalso represents the direction of lamination of the flexible sheets inthe flexible board 1 before bending.

The flexible board 1 is preferably configured to connect at least twohigh-frequency circuits and/or components within the housing of anelectronic device, such as a cell phone or a smartphone, for example. Inelectronic devices of recent years, various components and modules aredisposed with high density. To place the flexible board 1 in such anelectronic device, the flexible board 1 is bendable in accordance withthe shape of the space in which it is to be placed, and the flexibleboard 1 generally includes a sheet-shaped base 101 and connectors 103 aand 103 b, as shown in FIG. 1.

The base 101 preferably is made of a dielectric material, and has arectangular or substantially rectangular shape extending in the x-axisdirection when viewed in a plan view in the z-axis direction beforebending, as shown in the upper part of FIG. 2. More specifically, thebase 101 preferably is composed of flexible, thermoplastic resin, suchas polyimide or liquid crystal polymer, for example, so as to bebendable.

Here, the base 101 includes a pair of slits SL1 and SL2 and another pairof slits SL3 and SL4 provided in relation to bending positions. The slitSL1 is preferably configured to define a line parallel or substantiallyparallel to the x-axis direction, including a predetermined length 1 inthe x-axis direction and a predetermined width w (where w<<1) in they-axis direction, and extending completely through the base 101 in thez-axis direction. The predetermined length 1 is determined appropriatelyon the basis of the shape of the space in which the flexible board 1 isplaced.

The plane that passes through the center of the base 101 in the x-axisdirection and is parallel to the yz plane will be referred to herein asvertical center plane A-A′. Also, the plane that passes through thecenter of the base 101 in the y-axis direction and is parallel to the zxplane will be referred to herein as horizontal center plane B-B′. Theslit SL2 is plane-symmetrical to the slit SL1 with respect to horizontalcenter plane B-B′. In addition, the slits SL3 and SL4 areplane-symmetrical to the slits SL1 and SL2, respectively, with respectto vertical center plane A-A′.

Bending lines C, D1, D2, E, F1, and F2 are defined around the slits SL1and SL2. Bending line C is an imaginary line extending between the endsof the slits SL1 and SL2 that are located on the negative side in thex-axis direction. Bending line F1 is an imaginary line extending acrossthe shortest distance between the end of the slit SL1 that is located onthe positive side in the x-axis direction and one of the two edges ofthe base 101 that are parallel or substantially parallel to the x-axis,the edge being located on the negative side in the y-axis direction.Bending line F2 is an imaginary line positioned so as to beplane-symmetrical to bending line F1 with respect to horizontal centerplane B-B′. Bending line E is an imaginary line positioned on thepositive side in the x-axis direction relative to bending line C so asto be parallel or substantially parallel thereto at a predetermineddistance of Δa. Bending lines D1 and D2 are imaginary lines positionedon the negative side in the x-axis direction relative to bending linesF1 and F2, respectively, so as to be parallel or substantially parallelthereto at the predetermined distance of Δa.

Furthermore, bending lines G, H1, H2, I, J1, and J2 are defined aroundthe slits SL3 and SL4. Bending line G is an imaginary line positioned soas to be plane-symmetrical to bending line C with respect to verticalcenter plane A-A′. Bending lines J1 and J2 are imaginary linespositioned so as to be plane-symmetrical to bending lines F1 and F2,respectively, with respect to vertical center plane A-A′. Bending line Iis an imaginary line positioned so as to be plane-symmetrical to bendingline E with respect to vertical center plane A-A′. Bending lines H1 andH2 are imaginary lines positioned so as to be plane-symmetrical tobending lines D1 and D2, respectively, with respect to vertical centerplane A-A′.

The base 101 is bent along bending lines C, D1, D2, E, F1, and F2. Morespecifically, the base 101 is bent approximately 90° outward alongbending lines F1 and F2. Moreover, the base 101 is bent approximately90° outward along bending line E as well. Here, the “outward” bendingrefers to a bending method by which the bend protrudes toward thepositive side in the z-axis when it is viewed down from the positiveside in the z-axis direction toward the negative side. Further, the base101 is bent approximately 90° inward along bending lines D1 and D2.Moreover, the base 101 is bent approximately 90° inward along bendingline C as well. Here, the “inward” bending refers to a bending method bywhich the bend protrudes in the opposite direction to the “outward”bending, i.e., toward the negative side in the z-axis.

Furthermore, the base 101 is bent along bending lines G, H1, H2, I, J1,and J2 so as to possess symmetry with respect to vertical center planeA-A′.

The base 101 bent as described above is shown both in FIG. 1 and thelower portion of FIG. 2. The above bending results in the base 101including a first sheet section 105 a, second sheet sections 107 a and107 b, first bent sheet sections 109 a and 109 b, and second bent sheetsections 111 a to 111 d.

The first sheet section 105 a is an area enclosed by the two edges ofthe base 101 that are parallel or substantially parallel to the x-axis,bending lines E, F1, F2, I, J1, and J2, and the slits SL1 to SL4, andincludes a first principal surface parallel or substantially parallel tothe xy plane.

The second sheet section 107 a is an area enclosed by the two edges ofthe base 101 that are parallel or substantially parallel to the x-axis,one of the two edges of the base 101 that are parallel or substantiallyparallel to the y-axis, precisely, the edge being located on thenegative side in the x-axis direction, bending lines C, D1, and D2, andthe slits SL1 and SL2. The second sheet section 107 a includes a secondprincipal surface parallel to the xy plane at the distance of Δa fromthe first principal surface in the z-axis direction (i.e., in the normaldirection to the first principal surface).

Furthermore, the second sheet section 107 b is plane-symmetrical orsubstantially plane-symmetrical to the second sheet section 107 a withrespect to vertical center plane A-A′, and therefore, any detaileddescription thereof will be omitted.

The first bent sheet section 109 a is an area enclosed by the slits SL1and SL2 and bending lines C and E, and includes a third principalsurface not parallel to either the first principal surface or the secondprincipal surface. In the present embodiment, the third principalsurface is a plane perpendicular to both the first principal surface andthe second principal surface and parallel to the yz plane.

Furthermore, the first bent sheet section 109 b is plane-symmetrical orsubstantially plane-symmetrical to the first bent sheet section 109 awith respect to vertical center plane A-A′, and therefore, any detaileddescription thereof will be omitted.

The second bent sheet section 111 a is an area enclosed by one of thetwo edges of the base 101 that are parallel or substantially parallel tothe x-axis, precisely, the edge being located on the negative side inthe y-axis direction, bending lines D1 and F1, and the slit SL1. Thesecond bent sheet section 111 a has a fourth principal surface parallelto the third principal at a distance of (1-Δa) from the third principalsurface in the x-axis direction (i.e., in the normal direction to thethird principal surface).

Furthermore, the second bent sheet section 111 b is plane-symmetrical orsubstantially plane-symmetrical to the second bent sheet section 111 awith respect to horizontal center plane B-B′, and therefore, anydetailed description thereof will be omitted. Moreover, the second bentsheet sections 111 c and 111 d are plane-symmetrical or substantiallyplane-symmetrical to the second bent sheet sections 111 a and 111 b,respectively, with respect to vertical center plane A-A′, and therefore,any detailed descriptions thereof will be omitted.

Furthermore, in the above configuration, the first bent sheet section109 a is positioned between the second bent sheet sections 111 a and 111b in a plan view in the x-axis direction (i.e., the normal direction tothe third or fourth principal surface). Similarly, the first bent sheetsection 109 b is positioned between the second bent sheet sections 111 cand 111 d in a plan view in the x-axis direction. In this manner, thethird principal surface protrudes toward the negative side in the x-axisdirection relative to the fourth principal surface.

Furthermore, the base 101 includes at least one transmission line 113provided therein. The transmission line 113 is configured to transmit ahigh-frequency signal between the connectors 103 a and 103 b, andextends in the x-axis direction. In the present preferred embodiment,three tri-plate striplines 113 a to 113 c preferably are provided as thetransmission line 113, for example, as illustrated in FIG. 3. In thiscase, the base 101 is a multilayer board preferably formed by laminatingat least four flexible sheets 115 a to 115 d toward the positive side inthe z-axis direction.

The flexible sheets 115 a to 115 d are laminate members made ofthermoplastic resin as mentioned above, and have the slits SL1 to SL4provided therein. Moreover, the flexible sheets 115 a to 115 d are bentalong bending lines C to J2, as described above. Note that in FIG. 3,reference characters are assigned only to the slits and the bendinglines of the flexible sheet 115 d for the sake of convenience.

The flexible sheet 115 a preferably includes three ground conductors 117a to 117 c arranged in the y-axis direction on the principal surfacethat is located on the positive side in the z-axis direction (referredto below as the top surface).

Here, in the case of the flexible board 1, for example, the groundconductor 117 a is not provided across bending lines D1 and F1.Accordingly, the ground conductor 117 a is divided into three conductorportions in and around the second bent sheet section 111 a. In thepresent preferred embodiment, the ground conductor 117 a including aplurality of such conductor portions is considered as one entity. Thesame applies to the other ground conductors 117 b, 117 c and 117 e to117 g.

The ground conductors 117 a to 117 c preferably are made of, forexample, a metal material mainly composed of silver or copper andincluding a low specific resistance. More preferably, the groundconductors 117 a to 117 c are made of a metal foil mainly composed ofsilver or copper. Here, to allow the base 101 to be bent readily, it ispreferable that the ground conductors 117 a to 117 c not be provided onbending lines C, D1, D2, E, F1, and F2.

The flexible sheet 115 b preferably includes three signal lines 119 a to119 c arranged in the y-axis direction on the top surface. The signallines 119 a to 119 c have a narrower width than the ground conductors117 a to 117 c. Moreover, in a plan view in the z-axis direction, thesignal lines 119 a to 119 c are positioned between the two edges oftheir respectively corresponding ground conductors 117 a to 117 c thatare parallel or substantially parallel to the x-axis. The signal lines119 a to 119 c are made of a material similar to that of the groundconductors 117 a to 117 c. The signal lines 119 a to 119 c arepreferably configured for use in, for example, connecting an antenna andan RF circuit. The signal line 119 a is configured to transmit a signalin the frequency range for use in, for example, a first wireless LAN (5GHz). The signal line 119 b is configured to transmit a signal in thefrequency range for use in, for example, a second wireless LAN (2.4GHz). The signal line 119 c is configured transmit a signal in thefrequency range for use in, for example, a cellular network (700 MHz to2.7 GHz).

The flexible sheet 115 c is configured essentially in the same manner asthe flexible sheet 115 a, and preferably includes three groundconductors 117 e to 117 g provided on the top surface. The groundconductor 117 e is connected to the ground conductor 117 a bycorresponding interlayer connection conductors (not shown) in the firstsheet section 105 a, the second sheet sections 107 a and 107 b, and thesecond bent sheet sections 111 a and 111 c. The ground conductor 117 fis connected to the ground conductor 117 b by corresponding interlayerconnection conductors (not shown) in the first sheet section 105 a, thesecond sheet sections 107 a and 107 b, and the first bent sheet sections109 a and 109 b. The ground conductor 117 g is connected to the groundconductor 117 c by corresponding interlayer connection conductors (notshown) in the first sheet section 105 a, the second sheet sections 107 aand 107 b, and the second bent sheet sections 111 b and 111 d.

In the above configuration, the signal lines 119 a to 119 c arepositioned between the flexible sheets 115 b and 115 c. Moreover, theground conductor 117 a is exactly opposite to the ground conductor 117 ein the z-axis direction, with the flexible sheet 115 b, the signal line119 a, and the flexible sheet 115 c positioned therebetween. Similarly,the ground conductors 117 b and 117 c are exactly opposite to the groundconductors 117 f and 117 g, respectively, with the signal lines 119 band 119 c, etc., positioned therebetween.

The portion of the transmission line 113 shown in circle α, which islocated on the negative side in the x-axis direction relative tovertical center plane A-A′, has been described above. The transmissionline 113 possesses symmetry with respect to vertical center plane A-A′,and therefore, will not be described concerning the structure on thepositive side in the x-axis direction.

The connectors 103 a and 103 b are positioned at opposite ends of thebase 101, more specifically, at the ends that are located on thenegative and positive sides, respectively, in the x-axis direction. Theconnectors 103 a and 103 b are electrically connected to respective endsof the transmission line 113.

A non-limiting example of a method for producing the flexible board 1will be described below. While the following description focuses on oneflexible board 1 as an example, in actuality, large-sized flexiblesheets preferably are laminated and cut, so that a number of flexibleboards 1 are produced at the same time.

Prepared first are large-sized flexible sheets with their entire frontsurfaces copper-foiled. Next, via-holes are bored through predeterminedones of the large-sized flexible sheets by irradiating their bottomsurfaces (i.e., surfaces that are not copper-foiled) with laser beamswhere via-hole conductors are to be formed.

Next, ground conductors 117 a to 117 c are formed in one of thelarge-sized flexible sheets by photolithography, resulting in a flexiblesheet 115 a for an individual flexible board 1. Similarly, signal lines119 a to 119 c are formed on another large-sized flexible sheet,resulting in a flexible sheet 115 b for the individual flexible board 1.In addition, ground conductors 117 e to 117 g are formed in stillanother large-sized flexible sheet, resulting in a flexible sheet 115 cfor the individual flexible board 1. Further, land electrodes and wiringpatterns for mounting connectors 103 a and 103 b are formed on yetanother large-sized flexible sheet, resulting in a flexible sheet 115 dfor the individual flexible board 1.

Next, via-hole conductors are formed by filling the via-holes providedin the predetermined large-sized flexible sheets with a conductive pastemainly composed of copper.

Next, the large-sized flexible sheets are stacked in order, from thenegative side to the positive side in the z-axis direction: the flexiblesheets 115 a, 115 b, 115 c, and 115 d. The stacked flexible sheets arethen pressed from above and below for bonding. Subsequently, slits SL1to SL4 are provided in the bonded flexible sheets, and then, connectors103 a and 103 b are mounted on the sheets. Thereafter, the flexibleboard 1 is completed by bending in the manner as described above.

The flexible board 1 configured as described above electrically connectstwo high-frequency circuits within the housing of an electronic device,as mentioned earlier. FIG. 4A illustrates an example of an electronicdevice 200 to which the flexible board 1 is applied, as viewed in a planview in the y-axis direction. FIG. 4B illustrates the electronic device200 of FIG. 4A in a plan view from the negative side in the z-axisdirection.

In FIGS. 4A and 4B, the electronic device 200 includes the flexibleboard 1, circuit boards 202 a and 202 b, receptacles 204 a and 204 b, abattery pack (metallic body) 206, and a housing 210.

The housing 210 accommodates the flexible board 1, the circuit boards202 a and 202 b, the receptacles 204 a and 204 b, and the battery pack206.

The circuit board 202 a includes circuits (e.g., various passive andactive components), which constitute at least a portion of a firsthigh-frequency circuit 201. The first high-frequency circuit 201 isprovided with, for example, an antenna.

The circuit board 202 b includes circuits (e.g., various passive andactive components), which constitute at least a portion of a secondhigh-frequency circuit 203. An example of the second high-frequencycircuit 203 is a transmission or reception circuit.

The battery pack 206 is, for example, a lithium-ion secondary battery,and the surface thereof is wrapped by a metal cover.

The circuit board 202 a, the battery pack 206, and the circuit board 202b are arranged in this order, from the negative side to the positiveside in the x-axis direction.

The receptacles 204 a and 204 b are provided on the principal surfacesof the circuit boards 202 a and 202 b, respectively, that are located onthe negative side in the z-axis direction. The receptacles 204 a and 204b are connected to the connectors 103 a and 103 b of the flexible board1. Accordingly, signals transmitted between the circuit boards 202 a and202 b are applied to the connectors 103 a and 103 b via the receptacles204 a and 204 b. In this manner, the flexible board 1 connects thecircuit boards 202 a and 202 b.

Furthermore, the flexible board 1 is bent along each bending line (seeFIG. 2) so as to follow the surface profile of the battery pack 206 andcontact the battery pack 206 on the surface of the first sheet section105 a. In addition, in the example shown in FIGS. 4A and 4B, theprincipal surface of the first sheet section 105 a on the negative sidein the z-axis direction is fixed on the surface of the battery pack 206on the positive side in the z-axis direction by an adhesive or the like.

As is apparent from FIG. 11, in the conventional flexible board 50, thebent sheet sections 509 a and 509 b, which are thin and planar, areconnected to opposite ends of the first sheet section 505 a. In otherwords, each end of the first sheet section 505 a is supported bysubstantially one plane. Accordingly, the shape of the bent sheetsections 509 a and 509 b becomes unstable because of the weight and thesize of the first sheet section 505 a, shaking of the electronic device,etc., so that the signal lines in the bent sheet section 509 a might bepositioned closer to the signal lines in another sheet section or anexternal high-frequency circuit, causing crosstalk between them.

However, in the present preferred embodiment of the present invention, afirst end of the first sheet section 105 a is supported by the firstbent sheet section 109 a and the second bent sheet sections 111 a and111 b. Here, the first bent sheet section 109 a is situated in adifferent position from the second bent sheet sections 111 a and 111 bin the normal direction to the third principal surface (i.e., the x-axisdirection). Moreover, in a plan view in the normal direction to thethird principal surface, the first bent sheet section 109 a ispositioned between the second bent sheet sections 111 a and 111 b. Inother words, unlike in the conventional art, the first end of the firstsheet section 105 a is supported by three different planes. As with thefirst end, a second end of the first sheet section 105 a is supported bythree different planes, i.e., the first bent sheet section 109 b and thesecond bent sheet sections 111 c and 111 d. This configuration rendersit possible to significantly reduce or prevent changes in shape of thebent sheet sections 109 a, 109 b, and 111 a to 111 d due to the weightand the size of the first sheet section 105 a, shaking of the electronicdevice, etc. As a result, it is possible to inhibit the signal lines ineach sheet section from being positioned close to the signal lines inanother sheet section or an external high-frequency circuit, reducingcrosstalk with them. That is, it is possible to significantly reduce orprevent variations in high-frequency characteristics among individualflexible boards 1.

Assuming here that the dimension of the conventional first sheet section505 a in the x-axis direction is lx, and the maximum dimension of thefirst sheet section 105 a in the x-axis direction is also lx. In theexample of FIG. 1, the value of lx is equal to the distance between thefirst bent sheet sections 109 a and 109 b in the x-axis direction. Inthis case, the distance between the second bent sheet sections 111 a and111 c in the x-axis direction is less than lx. In addition, the distancebetween the second bent sheet sections 111 b and 111 d in the x-axisdirection is also less than lx. With this configuration, it is possibleto make the amount of deflection of the first sheet section 105 a lessthan conventional. Accordingly, the portions of the signal lines 119 ato 119 c that are positioned on the first sheet section 105 a are lesslikely to be positioned closer to other portions positioned on the firstbent sheet sections 109 a and 109 b and the second bent sheet sections111 a to 111 d. This also renders it possible to reduce the occurrenceof crosstalk. That is, it is possible to significantly reduce or preventvariations in high-frequency characteristics among individual flexibleboards 1.

Furthermore, in the above preferred embodiment, the signal lines 119 ato 119 c are configured so as to continue without breaks betweenopposite ends of the base 101 in the x-axis direction. On the otherhand, the ground conductors 117 a to 117 g are not provided on anybending lines. Here, in the case where a plurality of typical striplinesare arranged side by side, if ground conductors are discontinuous, it isdifficult to ensure isolation at such breaks in the ground conductors.However, in the flexible board 1 of the present preferred embodiment(after bending), the portions of the signal line 119 b that arepositioned on the first bent sheet sections 109 a and 109 b areseparated in the x-axis direction from both the portions of the signalline 119 a that are positioned on the second bent sheet sections 111 aand 111 c and the portions of the signal line 119 c that are positionedon the second bent sheet sections 111 b and 111 d. This configurationrenders it possible to reduce the occurrence of crosstalk between thesignal lines 119 a to 119 c even if the ground conductors 117 a to 117 gare not provided on the bending lines.

Note that in the above preferred embodiment, the transmission lines 113are exemplified by tri-plate striplines. However, this is not limiting,the three transmission lines 113 may be microstrip lines or tri-platestriplines in the first sheet section 105 a and the second sheetsections 107 a and 107 b and may be coplanar lines in the first bentsheet sections 109 a and 109 b and the second bent sheet sections 111 ato 111 d. More specifically, since the first bent sheet sections 109 aand 109 b and the second bent sheet sections 111 a to 111 d are bent,the coplanar structure in which conductors do not overlap in thethickness direction is preferable from the viewpoint of bendability.

Furthermore, the above preferred embodiment has been described withrespect to the example where the ground conductors are not provided onany bending line. However, this is not limiting, and the groundconductors can be provided on all bending lines except at least one.Moreover, each line does not have to include any ground conductor.Examples of such a line include a power line and an antenna line.

First Modification

In the above preferred embodiment, the base 101 of the flexible board 1(after bending) preferably possesses symmetry with respect to verticalcenter plane A-A′. However, this is not limiting, and the base 101 mayhave a characteristic structure as described in the preferredembodiment, only on one side, as in a flexible board 1 a shown in FIGS.5 and 6. More specifically, the first sheet section 105 a is supportedat one end by the first bent sheet section 109 a and the second bentsheet sections 111 a and 111 b, as shown in FIGS. 5 and 6. Moreover, thefirst bent sheet section 109 a is provided in a different position fromthe second bent sheet sections 111 a and 111 b in the normal directionto the third principal surface (i.e., the x-axis direction). Moreover,when viewed in a plan view in the normal direction to the thirdprincipal surface, the first bent sheet section 109 a is positionedbetween the second bent sheet sections 111 a and 111 b. On the otherhand, the first sheet section 105 a is connected at the other end to oneend of the second sheet section 107 b by a single bent sheet section301.

Second Modification

In the first modification, the second bent sheet sections 111 a and 111b of the flexible board 1 a (after bending) preferably are situated inthe same position with respect to the first bent sheet section 109 a inthe normal direction to the third principal surface. However, this isnot limiting, and the second bent sheet sections 111 a and 111 b may beprovided in different positions with respect to the first bent sheetsection 109 a in the normal direction to the third principal surface, asin a flexible board 1 b (after bending) shown in FIGS. 7 and 8.

In this manner, the first bent sheet section 109 a and the second bentsheet sections 111 a and 111 b are situated in different positions fromone another in the normal direction to the third principal surface, suchthat their positions do not coincide in the normal direction. As aresult, it is possible to further significantly reduce or preventchanges in shape of the bent sheet sections 109 a, 111 a, 111 b, and301. Thus, the occurrence of crosstalk is further reduced or prevented.

Third Modification

The above preferred embodiment has been described with respect to theflexible board 1, which is a so-called flat cable. However, the flatcable is merely an illustrative example, and the flexible board may beprovided in the form of a module (or a submodule) such as a flexibleboard 1 c shown in FIG. 9, in which surface-mount components 401 a and401 b, such as an IC chip and passive components, are mounted on thefirst sheet section 105 a and the second sheet section 107 a included inthe base 101. FIG. 10 provides plan views of the flexible board 1 c asseen in the z-axis direction, where the upper and lower portions showthe flexible board 1 c before and after bending, respectively.

As described above, this configuration is resistant to deformation ofthe first bent sheet section 109 a and the second bent sheet sections111 a and 111 b, and therefore, even when the components 401 a and 401 bare mounted on the first sheet section 105 a and the second sheetsection 107 a, it is possible to prevent the occurrence of crosstalk aswell as deformation of the base 101 under the weight of the component401 a, etc.

Note that in the above preferred embodiment and modifications, the firstbent sheet sections 109 a and 109 b, when viewed in a plan view from thepositive side in the z-axis direction, preferably protrude so as to beconvex with respect to the first sheet section 105 a. However, this isnot limiting, and the first bent sheet sections 109 a and 109 b may berecessed so as to be concave with respect to the first sheet section 105a.

Although the present invention has been described in connection with thepreferred embodiments and modifications thereof above, it is to be notedthat various changes and modifications are possible to those who areskilled in the art. Such changes and modifications are to be understoodas being within the scope of the invention.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A flexible board comprising: a first sheetsection including a first principal surface; a second sheet sectionincluding a second principal surface and provided in a differentposition from the first principal surface in a normal direction to thefirst principal surface; a first bent sheet section configured toconnect ends of the first and second sheet sections, the first bentsheet section including a third principal surface not parallel to thefirst and second principal surfaces; a second bent sheet sectionincluding a fourth principal surface and provided in a differentposition from the third principal surface in a normal direction to thethird principal surface; a signal line provided in or on the first sheetsection, the second sheet section, and one of the first bent sheetsection and the second bent sheet section; and a ground conductorprovided in or on the first sheet section, the second sheet section, andthe one of the first bent sheet section and the second bent sheetsection, so as to be parallel or substantially parallel to the signalline; wherein the ground conductor is disposed so as not to bepositioned at a connection between the first sheet section and the firstbent sheet section and a connection between the second sheet section andthe first bent sheet section or so as not to be positioned at aconnection between the first sheet section and the second sheet bentsection and a connection between the second sheet section and the secondbent sheet section.
 2. The flexible board according to claim 1, furthercomprising a signal line provided in or on the first sheet section, thesecond sheet section, and the other one of the first bent sheet sectionand the second bent sheet section.
 3. The flexible board according toclaim 1, wherein the first principal surface and the second principalsurface are parallel or substantially parallel to each other.
 4. Theflexible board according to claim 2, wherein the first principal surfaceand the second principal surface are parallel or substantially parallelto each other.
 5. The flexible board according to claim 1, wherein thefirst sheet section, the second sheet section, the first bent sheetsection, and the second bent sheet section are defined by bending oneflexible base.
 6. An electronic device comprising: the flexible boardaccording to claim 1; and at least two high-frequency circuits connectedby the flexible board.